The correct answer is C. 40 %.
Invar is a nickel-iron alloy with a very low coefficient of thermal expansion. This makes it ideal for use in measuring base lines, which are used to establish the positions of surveying markers. The nickel content of Invar is typically 40%, which is the optimal amount for achieving the desired low coefficient of thermal expansion.
Option A is incorrect because it is the percentage of nickel in a different alloy, Inconel. Inconel is a nickel-chromium alloy that is used in a variety of applications where high strength and corrosion resistance are required.
Option B is incorrect because it is the percentage of nickel in a third alloy, Monel. Monel is a nickel-copper alloy that is used in a variety of applications where high strength and corrosion resistance are required.
Option D is incorrect because it is a higher percentage of nickel than is typically used in Invar. A higher percentage of nickel would result in a higher coefficient of thermal expansion, which would make Invar less suitable for use in measuring base lines.